Analysis of parity-violating asymmetries in elastic electron-nucleus scattering

Parity violating asymmetries in elastic electron scattering on nuclei with arbitrary spin and isospin are considered in detail, extending our previous work on this matter [1]. Examples are given for the target nuclei¹²C,⁴⁰Ca,²⁰⁸Pb,³H,³He,⁴¹Ca,²⁰⁹Bi. Several ways of extracting information on the structure of weak neutral current couplings are presented. Besides, as a further possibility to check the validity of the standard gauge model [2] at intermediate energies, the chances to distinguish among different classes of gauge models are discussed.     

On the Pringsheim Rearrangement Theorems

We show that requiring that the set of positions of the positive terms in a conditionally convergent numerical series have asymptotic density provides converses for old rearrangement theorems of Alfred Pringsheim.     

Bridging-ligand-facilitated intramolecular communication in homo- and heterobimetallic transition metal complexes of copper(I) and silver(I) as evidenced by cyclic voltammetry

Ohne Zusammenfassung     

Ansa-Steroids

The backbone of steroids is broken by a cycloaddition-retro-reaction sequence of propargylic aldehyde to generate a 14-membered ansa compound.     

Copper-bispidine coordination chemistry: syntheses,structures, solution properties,and oxygenation reactivity

Copper(I) and copper(II) complexes of two mononucleating and four dinucleating tetradentate ligands with a bispidine backbone (2,4-substituted (2-pyridyl or 4-methyl-2-pyridyl) 3,7-diazabicyclo[3.3.1]nonanone) have been prepared and analyzed structurally, spectroscopically, and electrochemically. The structures of the copper chromophores are square pyramidal, except for two copper(I) compounds which are four-coordinate with one noncoordinated pyridine. The other copper(I) structures have the two pyridine donors, the co-ligand (NCCH(3)), and one of the tertiary amines (N3) in-plane with the copper center and the other amine (N7) coordinated axially (Cu-N3 > Cu-N7, approximately 2.25 A vs 2.20 A). The copper(II) compounds with pyridine donors have a similar structure, but the axial amine has a weaker bond to the copper(II) center (Cu-N3 < Cu-N7, approximately 2.03 A vs 2.30 A). The structures with methylated pyridine donors are also square pyramidal with the co-ligands (Cl(-) or NCCH(3)) in-plane. With NCCH(3) the same structural type as for the other copper(II) complexes is observed, and with the bulkier Cl(-) the co-ligand is trans to N7, leading to a square pyramidal structure with the pyridine donors rotated out of the basal plane and only a small difference between axial and in-plane amines (2.15, 2.12 A). These structural differences, enforced by the rigid bispidine backbone, lead to large variations in spectroscopic and electrochemical properties and reactivities. Oxygenation of the copper(I) complexes with pyridine-substituted bispidine ligands leads to relatively stable mu-peroxo-dicopper(II) complexes; with a preorganization of the dicopper chromophores, by linking the two donor sets, these peroxo compounds are stable at room temperature for up to 1 h. The stabilization of the peroxo complexes is to a large extent attributed to the square pyramidal coordination geometry with the substrate bound in the basal plane, a structural motif enforced by the rigid bispidine backbone. The stabilities and structural properties are also seen to correlate with the spectroscopic (UV-vis and Raman) and electrochemical properties.     

Modifizierung von Schichtsilicaten mit sterisch anspruchsvollen Metallkomplexen. 3. Synthese und Intercalation der planar-quadratischen Komplexe [Cu(bppep)(H2O)](ClO4)2 und [Ni(bppep)(Cl)Cl] (bppep = 2,6-Bis[1-phenyl-1-(pyridin-2-yl)ethyl]pyridin) in Hectorit

Modification of Layer Silicates by Sterically Demanding Metal Complexes: Synthesis and Intercalation of the Square Planar Complexes [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl (bppep = 2,6-Bis[1-phenyl-1-(pyridine-2-yl)ethyl]pyridine) in Hectorite Sodium-aqua hectorite reacts with [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl with exchange of the sodium-aqua cations against the complex cations [Cu(bppep)(H₂O)]²⁺ and [Ni(bppep)(Cl)]⁺, respectively. In addition, cation-anion pairs of [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl are also intercalated between the hectorite layers (intersalation). On the other hand, it is possible to synthesize [Cu(bppep)(H₂O)]²⁺ or [Ni(bppep)(H₂O)]²⁺ modified hectorites without additional ion-pair intercalation (intersalation) by reaction of nickel- and copper-hectorites with the bppep ligand.     

Octaisopropylchromocen: Das erste Chromocen mit kontinuierlichem low spin/high spin-Übergang

Octaisopropylchromocene: The First Chromocene with Gradual low spin/high spin-Transition Octaisopropylchromocene ( 1 ) exists in a low spin state at very low temperatures and undergoes a gradual spin transition on warming up. At 300 K a high spin state with four unpaired electrons has been found for solid 1 by investigation of its magnetic properties.     

Inter- and intraspecific variation in excited-state triplet energy transfer rates in reaction centers of photosynthetic bacteria

In protein-cofactor reaction center (RC) complexes of purple photosynthetic bacteria, the major role of the bound carotenoid (C) is to quench the triplet state formed on the primary electron donor (P) before its sensitization of the excited singlet state of molecular oxygen from its ground triplet state. This triplet energy is transferred from P to C via the bacteriochlorophyll monomer B(B). Using time-resolved electron paramagnetic resonance (TREPR), we have examined the temperature dependence of the rates of this triplet energy transfer reaction in the RC of three wild-type species of purple nonsulfur bacteria. Species-specific differences in the rate of transfer were observed. Wild-type Rhodobacter capsulatus RCs were less efficient at the triplet transfer reaction than Rhodobacter sphaeroides RCs, but were more efficient than Rhodospirillum rubrum RCs. In addition, RCs from three mutant strains of R. capsulatus carrying substitutions of amino acids near P and B(B) were examined. Two of the mutant RCs showed decreased triplet transfer rates compared with wild-type RCs, whereas one of the mutant RCs demonstrated a slight increase in triplet transfer rate at low temperatures. The results show that site-specific changes within the RC of R. capsulatus can mimic interspecies differences in the rates of triplet energy transfer. This application of TREPR was instrumental in defining critical energetic and coupling factors that dictate the efficiency of this photoprotective process.     

Intramolecular cyclizations of allyl- and propargylsilanes

Allylsilanes of type $\text{5}$ and propargylsilanes of type $\text{13}$ undergo efficient cyclizations upon treatment with EtAlCl₂ in toluene to afford stereoselectively functionalized hydrindanones of type $\text{6}$ and spiro[4,5]decanes of type $\text{14}$.